Bond coats are applied over substrate base metals used in gas turbine engines. These bond coats are applied for various purposes, having different compositions, the composition depending upon its purpose. When used on components located in the hot section of an aircraft engine, the turbine portion, these bond coats are used for environmental protection and as a surface for application of a thermal barrier coating (TBC) that enhances the adhesion of the TBC to the component.
Many different varieties of bond coats have been devised, frequently differing only subtlety in composition, microstructure, method of application or mechanical feature. One common problem is that applied thermal barrier coatings tend to spall from the surface of the component. This can occur for any one of a variety of reasons. For example, one identified problem has been the adverse effect of sulfur from the complex substrate chemistry, typically a superalloy material, into the bond coat, which was associated with premature spallation of applied TBC. While many problems with different proposed solutions have been identified, the frequently identified problem to be solved is the retention of TBC on a turbine component in the hot, environmentally deleterious environment in the turbine section of a gas turbine engine such as an aircraft engine.
Resulting changes in the chemistry of both the substrate and the bond coat due to diffusion of elements from the complex substrate chemistry, typically a superalloy material, into the bond coat at the elevated temperatures of turbine operation remains problematic. These coatings frequently are aluminum bearing materials from the class of diffusion aluminides, including modified versions that include platinum-type metals and overlay aluminides such as NiAlCrZr and MCrAlY (where M is at least one of Ni, Co and Fe). These coatings are designed to take advantage of the excellent environmental properties of alumina or aluminum oxide (Al2O3) which forms at the surface. However, the capability of such environmental coatings is generally limited by the depletion of the active elements, typically aluminum through its continued oxidation at the surface as spalling and diffusion of the aluminum into the substrate occurs. But, diffusion of aluminum into the substrate can also result in detrimental secondary reaction zones (SRZs) forming in newly-developed superalloys. The capability of the coating as a bond coat for a TBC is also limited by a thickening of the thermally grown diffusion layer between the TBC and the environmental/bond coat and spallation due to thermally-induced stresses.
Solutions to this problem include formation of a diffusion barrier layer between the superalloy substrate and the bond coat and application of a platinum-group metal (PGM) bond coating over the base metal with or without a diffusion barrier layer. Because the chemical compositions of the bond coat and the superalloy substrate are significantly different, the driving forces (activation energies) for diffusion between superalloy substrates and metallic bond coats at the elevated temperatures of operation are very strong, so the solution is to prevent or significantly reduce the diffusion of elements from one to the other. U.S. Pat No. 6,933,052to Gorman et al. issued Aug. 23, 2005, assigned to the assignee of the present invention, is an example of such a solution, utilizing a diffusion barrier between the bond coat and the substrate which inhibits the diffusion of elements from the substrate to the bond coat. The diffusion barrier, a non-metallic oxide or nitride intermediate between the substrate and bond coat and overlying the bond coat, inhibits the diffusion of elements between the bond coat and substrate. The diffusion of elements across the diffusion barrier from either the superalloy substrate or the bond coat is significantly lowered. One of the limiting features of these solutions when using a PGM-based coating is continued diffusion of oxygen resulting in oxidation of either or both of the underlying base metal or diffusion barrier layer, the diffusion of oxygen continuing even while diffusion of other elements comprising the superalloy substrate or bond coat is lowered.
When a diffusion barrier is not utilized, a further limitation is caused by extensive Kirkendall voiding that is observed from the rapid interdiffusion of the PGM-based bond coat and the substrate. Coalescence of significant Kirkendall voids can result in spallation of the overlying protective bond coating, leaving the substrate material unprotected. Diffusion barriers that have been utilized have critical adhesion limitations with the substrate or limited environmental resistant that cannot tolerate oxygen ingress or any breach in the coating.
The present invention is designed to reduce the diffusion between a bond coat and a superalloy substrate, but utilizes a different approach.